Modular filters for television receivers

ABSTRACT

A modular filter for separate attachment to receivers between the tuner and the intermediate frequency stages comprising a compact housing containing passive electronic components connected in circuit with input and output connectors to provide electromagnetic wave filtering at variably selected frequencies.

UllllCu DlalCB l'flltilll Asplin Mar. 27, 1973 {54] MODULAR FILTERS FORTELEVISION References Cmd RECEIVERS UNITED STATES PATENTS 1 lnvemoflWilliam P 6162 Ridgwood 2,650,265 8/1953 Mountjoy ..l78/5.2 A Dr. S.W.,Canton, Ohio 44706 2,901,533 8/1959 Keizer ..178/5.2 A Filed: 1971Primary Examiner-Howard W. Britton Au -W'l & F 21 Appl. No.: 104,032

1 [57] ABSTRACT Cl "325/477, 173/016- 325/308 A modular filter forseparate attachment to receivers 325/355 between the tuner and theintermediate frequency [5 1] Int. Cl. ..l-l04b 1/10, H04n 7/18 stagescomprising a compact housing containing pas- Field 3 sive electroniccomponents connected in circuit with 325/355, 308 input and outputconnectors to provide electromagnetic wave filtering at variablyselected frequencies.

1 Claim, 4 Drawing Figures i r P *x! Patented March 27, 1973 2SheetsSheet 1 VIDEO 45.75

47.25 UPPER SOJND 48.|7 UPPER COLOR 5|.75 UPPER VIDEO FREQUENCY F I G. l

FREQUENCY (MHz) F I. G. 4

W ILLIAM N ATTORNEYS Patented March 27, 1973.

2 Sheets-Sheet 2 FIG.Z

OUTPUT a c 4/ 3 3 T Y]. gamma LT 6 1k |C5 Q/ W INPUT FIG.3

INV EN TOR.

A ASPLI N WILLIAM A TTORNEYS MODULAR FILTERS FOR TELEVISION RECEIVERSBACKGROUND OF THE INVENTION The concept of utilizing transmission linesto distribute broadcast quality television signals directly to homeviewers has been known for many years and is referred to as cabletelevision or cable TV as contrasted with conventional or free air"reception. There have been a large number of technical and economicproblems encountered in the development of cable television and someremain yet unsolved. In order to be economically feasible cabletelevision receivers must be compatible and interchangeable for use asfree air receivers so that a prospective user of cable television willnot be forced to acquire a separate television receiver for cablereception.

A large number of commercially available television receivers are notsuitable for use in cable systems because of their poor selectivitywhich is the result of inadequate filtering of the television signal.Selectivity is the name given to the ability of a receiver todistinguish or select a particular signal from other signals nearby inthe frequency spectrum. Good selectivity is an essential characteristicof receivers used in a cable than in the conventional broadcastspectrum.

SUMMARY OF THE INVENTION Accordingly, it is the purpose of thisinvention to provide an attachment for conventional television receiverswhich will improve their selectivity characteristics and permit theiruse in cable systems. The attachment of the invention is a modularfilter which may be inserted between the tuner and the intermediatefrequency amplifier stages by a qualified repairman in the home of theset owner without removing the set to the shop. The filter is a passivedevice comprising bandpass or band-reject circuitry contained within acompact housing having input and output connections. The filter isconnected into the circuitry by opening the signal path from the tunerto the intermediate frequency stages and inserting the filter in series.By using conventional connectors and connecting cables the filter may beremoved or replaced as desired.

DESCRIPTION OF THE DRAWINGS:

DESCRIPTION OF A PREFERRED EMBODIMENT The invention will be understoodmore readily by referring to the drawing in connection with thefollowing explanation. FIG. 1 is a graph of signal amplitude versusfrequency for the tuner output on channel 3 of a typical televisionreceiver. The desired sound and video carrier signals are shown as beingof maximum amplitude while the upper and lower sound and video signalsof adjacent channels are shown as attenuated in varying degrees by thefilter arrangement of the receiver. In order to provideinterference-free viewing all carriers except the desired sound andvideo carriers must be attenuated at the input to the intermediatefrequency stages. The degree of attenuation required is on the order of20 db for vacuum tube stages and 25 to 30 db for transistor stages.

The most troublesome of the carrier frequencies shown in FIG. 1 are39.75 MHz, 47.25 MHz and 48.17 MHz. The rejection pattern exhibited bythe degree of attenuation of these frequencies as shown is insufficientfor proper reception when this receiver is to be used in a cable system.This rejection pattern is typical of many commercially availabletelevision receivers. If a set exhibiting these characteristics were tobe used in a cable system an improved filtering arrangement would haveto be provided to insure adequate reception.

In accordance with the present invention a passive filter device inmodular form is inserted between the tuner and the intermediatefrequency amplifier stages of a television receiver to eliminateundesirable frequencies. FIG. 2 shows a conventional television tuner Iwhich normally feeds a combined sound and picture signal to theintermediate frequency stages 3. The showing here is simplified forpurposes of clarity, and those skilled in the art will appreciate thatin actual practice the physical configurations may differ somewhat.

The filter device of the invention is shown at 5 and comprises a housingwith input and output connectors mounted thereon. The output connectoris shown at 7 and may be any suitable connector which provides adequatesignal shielding. Tuner 1 is provided with an output connector 9, andthe intermediate frequency stages 3 receive the input signal throughinput connector I I. The input connector on filter 5 is not shown inFIG. 2. The tuner I, filter 3 and intermediate frequency stages 3 areconnected together by suitable interconnecting cables 13 and 15 whichare provided with connectors complimentary to the connectors 7, 9 and lI.

The electrical circuitry of filter 5 is shown schematically in FIG. 3.The component values for this circuit are as follows:

C, I 8-50 mifd. variable C 5 )Lpfd.

C,- 8 mifd. C. =5-25 mfd. variable C.- 5 #pfd. C. 5-25 pufd. variableC.- 4.7 upfd. T 7,, T T. l.9 2.4 mh

variable tained by introducing the filter 5 into the circuit.

In the circuit of FIG. 3 series resonant inductance T, and capacitanceC, and parallel resonant inductance T and capacitances C. and C act asband pass elements and attenuate by at least 18 db all frequenciesoutside of the desired band which in this case is 41.25 MHz to 46.25MHz. Series resonant traps formed by the ining, mounted within. saidhousing and connected between said input and output connectors toprovide a plurality of series resonant circuits and at least oneparallel resonant circuit to produce electromagnetic wave filtering atdesired frequencies, means for connecting the input connector to theoutput of said tuner, and means for connecting the output connector tothe input of said intermediate frequency stages, whereby theconventional television receiver will have greatly improved selectivityand will be adaptable for use in cable television systems.

i m t w s

1. A modular filter attachment for cable television for mounting in aconventional television receiver between the tuner and the intermediatefrequency stages comprising a housing member, input and outputconnectors mounted on said housing member, a plurality of passivecomponents, at least some of which are variable in value to permitresonant tuning, mounted within said housing and connected between saidinput and output connectors to provide a plurality of series resonantcircuits and at least one parallel resonant circuit to produceelectromagnetic wave filtering at desired frequencies, means forconnecting the input connector to the output of said tuner, and meansfor connecting the output connector to the input of said intermediatefrequency stages, whereby the conventional television receiver will havegreatly improved selectivity and will be adaptable for use in cabletelevision systems.